Implement UTCTime decoding

Remove LocalTime decoding because it requires IO to convert from Postgres time.

lib/Database/PostgreSQL/Opium/FromField.hs

@@ -19,8 +19,8 @@ import Data.Proxy (Proxy (..))
 import Data.Time
   ( Day (..)
   , DiffTime
-  , LocalTime (..)
   , TimeOfDay
+  , UTCTime (..)
   , addDays
   , fromGregorian
   , picosecondsToDiffTime
@@ -171,23 +171,20 @@ instance FromField TimeOfDay where
   validOid Proxy = Oid.time
   parseField = timeToTimeOfDay <$> parseField @DiffTime
 
+fromPostgresTimestamp :: Int -> (Day, DiffTime)
+fromPostgresTimestamp ts = (day, time)
+  where
+    (days, microseconds) = ts `divMod` (86400 * 1000000)
+    day = fromPostgresJulian $ fromIntegral days
+    time = picosecondsToDiffTime $ fromIntegral microseconds * 1000000
+
 -- | See https://www.postgresql.org/docs/current/datatype-datetime.html.
--- | Accepts @timestamp@.
--- Note that Postgres uses the proleptic Gregorian calendar, whereas @Show Day@ and @fromGregorian@ use an astronomical calendar.
--- In short, Postgres treats 1 BC as a leap year and doesn't have a year zero.
--- This means that working with negative dates will be different in Postgres and your application code.
-instance FromField LocalTime where
-  validOid Proxy = Oid.timestamp
-  parseField = fromPostgresTimestamp <$> readBigEndian @Int <$> AP.takeByteString
+-- Accepts @timestamp with timezone@.
+instance FromField UTCTime where
+  validOid Proxy = Oid.timestampWithTimezone
+  parseField = toUTCTime . fromPostgresTimestamp <$> readBigEndian @Int <$> AP.takeByteString
     where
-      fromPostgresTimestamp :: Int -> LocalTime
-      fromPostgresTimestamp ts =
-        let
-          (days, microseconds) = ts `divMod` (86400 * 1000000)
-          day = fromPostgresJulian $ fromIntegral days
-          time = picosecondsToDiffTime $ fromIntegral microseconds * 1000000
-        in
-          LocalTime day (timeToTimeOfDay time)
+      toUTCTime (day, time) = UTCTime day time
 
 newtype RawField a = RawField a
   deriving (Eq, Show)

lib/Database/PostgreSQL/Opium/Oid.hs

@@ -60,3 +60,7 @@ time = eq $ Oid 1083
 -- | A point in time.
 timestamp :: Oid -> Bool
 timestamp = eq $ Oid 1114
+
+-- | A point in time.
+timestampWithTimezone :: Oid -> Bool
+timestampWithTimezone = eq $ Oid 1184